Side-hole cane waveguide sensor

ABSTRACT

A side-hole optical cane for measuring pressure and/or temperature is disclosed. The side-hole cane has a light guiding core containing a sensor and a cladding containing symmetrical side-holes extending substantially parallel to the core. The side-holes cause an asymmetric stress across the core of the sensor creating a birefringent sensor. The sensor, preferably a Bragg grating, reflects a first and second wavelength each associated with orthogonal polarization vectors, wherein the degree of separation between the two is proportional to the pressure exerted on the core. The side-hole cane structure self-compensates and is insensitive to temperature variations when used as a pressure sensor, because temperature induces an equal shift in both the first and second wavelengths. Furthermore, the magnitude of these shifts can be monitored to deduce temperature, hence providing the side-hole cane additional temperature sensing capability that is unaffected by pressure. Additionally, the side-hole cane can be used to measure a differential pressure between a first pressure ported to the side-holes and a second external pressure.

CROSS-REFERENCES TO RELATED APPLICATIONS

[0001] This application contains subject matter related to thatdisclosed in U.S. patent application Ser. No. ______ (Attorney DocketNumber CC-0283), filed ______ and entitled ______; and PCT PublicationNo. WO01/40835, published Jun. 7, 2001, publishing PCT Application No.PCT/US00/32934, entitled “Large Diameter Optical Waveguide, Grating, andLaser,” filed Dec. 5, 2000, which claims priority to U.S. patentapplication Ser. No. 09/455,868, filed Dec. 6, 1999, which areincorporated herein by reference in their entirety.

TECHNICAL FIELD

[0002] This invention relates generally to birefringent cane sensors,and more particularly, to a birefringent cane sensor having symmetricalside-holes for pressure and temperature sensing.

BACKGROUND ART

[0003] Birefringent optical fibers for sensing parameters such aspressure are known in the art. In a birefringent fiber, birefringence iscaused in part by the geometrical asymmetry that occurs when the opticalfiber deforms under strain. However, because optical fibers are made ofglass, and are typically fragile and small, they are relativelydifficult to deform, which limits the sensitivity of such sensors.Typical fiber sensors, such as those made from standard fiber opticcommunication cables, have outer diameters in the range of 125 micronswith optical cores of 7 to 12 microns and therefore have relatively lowclad-to-core ratios.

[0004] Side-holes have been incorporated into fiber as is disclosed, forexample, in U.S. Pat. No. 6,208,776, entitled “Birefringent FiberGrating Sensor and Detection System,” which is incorporated herein byreference. By incorporating side-holes into the cladding of the fiber,the fiber's mechanical compliancy is increased as well as its potentialsensitivity and range when used as a sensor. Birefringent fibers havealso in the prior art incorporated specific sensing elements such asBragg gratings for measuring desired parameters including pressure andtemperature, as is disclosed in U.S. Pat. No. 6,304,686, entitled“Methods and Apparatus for Measuring Differential Pressure with FiberOptic Sensor Systems,” which is incorporated herein by reference.

[0005] However, fiber optic based birefringent sensors are limited bytheir physical characteristics and manufacturing difficulties. Forexample, fiber sensors may not be subject to large pressures parallel tothe axis of the fibers because the fibers may buckle. Additionally,fibers are small and delicate, and require special care during handlingand manufacturing. Additionally, the protective buffer coating typicallyformed on standard optic cable has to be contented during manufacturingas one skilled in the art will understand, which adds manufacturingcomplexity and hence extra time and cost. Manufacturing yields forstandard fiber-based sensors containing gratings can be lower than 10%,which is clearly not optimal. Formation of the side-holes in therelatively small cladding of the fiber can also be difficult toaccomplish.

[0006] A waveguide that has been used to counteract some of thedifficulties associated with optical “fibers” is a waveguide with adiameter ranging from about 0.3 mm to 4 mm, referred to as a “cane.”Cane waveguides have a core and a cladding just as do standard fibers.In fact, the core of a single mode cane is generally the same diameteras the core of a single mode standard fiber, typically 7 to 12 microns.However, cane is thicker and sturdier than fiber because of thesubstantial amount of cladding. While a standard fiber has a diameter of125 microns, cane ranges from 0.3 mm to about 4 mm, the great bulk ofwhich constitutes cladding. The cane's relatively thick claddingprovides significant mechanical benefits over fiber. Furthermore, a canedoes not require a protective buffer layer, and thus eliminatesmanufacturing complexity.

[0007] The art would benefit from ways to improve the performance ofpressure and temperature sensing in a side-hole fiber by utilizing thestructure of a cane. Such an improvement is disclosed herein,specifically a cane-based side-hole sensor which has improvedsensitivity, is easier to manufacture, handle, and package, is moreresilient, and which otherwise substantially eliminates the shortcomingsof fiber-based side-hole sensors. In particular, the art of oil/gasproduction would especially benefit from improved pressure sensorsutilizing sturdier cane-based structures which are suitable fordeployment in harsh environments such as oil/gas wells.

SUMMARY OF THE INVENTION

[0008] A side-hole optical cane for measuring pressure and/ortemperature is disclosed. The side-hole cane has a light guiding corecontaining a sensor and a cladding containing symmetrical side-holesextending substantially parallel to the core. The side-holes cause anasymmetric stress across the core of the sensor creating a birefringentsensor. The sensor, preferably a Bragg grating, reflects a first andsecond wavelength each associated with orthogonal polarization vectors,wherein the degree of separation between the two is proportional to thepressure exerted on the core. The side-hole cane structureself-compensates and is insensitive to temperature variations when usedas a pressure sensor, because temperature induces an equal shift in boththe first and second wavelengths. Furthermore, the magnitude of theseshifts can be monitored to deduce temperature, hence providing theside-hole cane additional temperature sensing capability that isunaffected by pressure. Additionally, the side-hole cane can be used tomeasure a differential pressure between a first pressure ported to theside-holes and a second external pressure.

[0009] The foregoing and other objects, features, and advantages of thepresent disclosure will become more apparent in light of the followingdetailed description of exemplary embodiments thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a side view perspective of a standard cane waveguide asis known.

[0011]FIG. 2 is a side view perspective of a side-hole cane, inaccordance with one embodiment of the present invention.

[0012]FIG. 3 is a cross-sectional view of a solid cane and a side-holecane with beveled edges to facilitate their connection, in accordancewith one embodiment of the present invention.

[0013]FIG. 4 is a cross-sectional view of a solid cane and a side-holecane joinable by a protrusion/pocket arrangement, in accordance with oneembodiment of the present invention.

[0014]FIG. 5 is a side view perspective of a side-hole cane containing aBragg grating, in accordance with one embodiment of the presentinvention.

[0015]FIG. 6 is an exploded side view of a core containing a Bragggrating, in accordance with one embodiment of the present invention.

[0016]FIG. 7 is a cross-sectional view of a side-hole cane disposedbetween two pieces of solid cane, in accordance with one embodiment ofthe present invention.

[0017]FIG. 8 is a cross-sectional view of a side-hole cane disposedbetween two pieces of standard fiber, in accordance with one embodimentof the present invention.

[0018]FIG. 9 is a cross-sectional view of a side-hole cane surrounded bya housing partitioned into two chambers for measuring a differentialpressure, in accordance with one embodiment of the present invention.

[0019]FIG. 10 is a cross-sectional view of a side-hole cane partiallyenclosed by a housing for measuring a differential pressure, inaccordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0020] In the disclosure that follows, in the interest of clarity, notall features of actual commercial implementations of a side-hole canesensor and related techniques are described. It will of course beappreciated that in the development of any such actual implementation,as in any such project, numerous engineering and design decisions mustbe made to achieve the developers' specific goals, e.g., compliance withmechanical and business related constraints, which will vary from oneimplementation to another. While attention must necessarily be paid toproper engineering and design practices for the environment in question,it should be appreciated that development of a side-hole cane sensor andrelated techniques would nevertheless be a routine undertaking for thoseof skill in the art given the details provided by this disclosure, evenif such development efforts are complex and time-consuming.

[0021] Referring to FIG. 1, a large diameter “cane” optical waveguide 20has at least one core 14 surrounded by a thick cladding 16. Knownprocesses such as vapor phase deposition or direct-melt methods mayproduce the cane structure, such as is disclosed in U.S. patentapplication Ser. No. 09/455,868, filed Dec. 6, 1999 and entitled “LargeDiameter Optical Waveguide, Grating, and Laser,” which is incorporatedherein by reference in its entirety. The cane waveguide 20 preferablycomprises silica glass (SiO₂) based material having appropriate dopants,as is known, to allow light 11 to propagate through the core 14. Othermaterials for the cane waveguide 20 may be used if desired. For example,the cane waveguide 20 may be made of any glass, such as phosphate,aluminosilicate, borosilicate, fluoride glasses or other glasses, or maybe made of plastic. The cane waveguide 20 may be made using fiberdrawing techniques now known or later developed that provide theresultant desired dimensions for the core 14 diameter D₁ and the outerdiameter D₂. The external surface of the cane waveguide 20 is preferablyoptically non-distorting, thereby allowing Bragg gratings to be writtenthrough the cladding 16 in a manner similar to that used for writinggratings into a conventional optical fiber 22.

[0022] As alluded to earlier, the clad-to-core diameter ratio of thecane 20 is exceptionally large when compared to fiber, typically rangingfrom about 30 to 1 to 300 to 1. (Fiber, by contrast, has a clad-to-coreratio of approximately 12 to 1). This substantial cladding 16distinguishes a “cane” 20 from a standard “fiber” (such as standard 125micron diameter communications fiber) and provides significant benefitsin side-hole-based applications and manufacturing, as will be describedin more detail below. The cladding 16 preferably has an outer diameterD₂ of at least 0.3 mm and generally about 4 mm or more, and the core 14preferably has an outer diameter D₁ of about 7 to 12 microns (such thatit propagates only a single mode at or above the cutoff wavelength and afew (e.g., six or less) spatial modes below the cutoff wavelength as isknown). The outer diameter D₂ of the cladding 16 and the length L aretypically matched such that the cane 20 will resist buckling when placedin compression along the core's axis. By contrast, optical fiber is moreeasily susceptible to buckling, especially under the influence ofstresses parallel to the core of the fiber, due to the largerlength-to-diameter aspect ratios usually found in fiber-based sensors.

[0023] In addition to resisting buckling, the cane 20 has inherentmechanical rigidity which makes it easier to handle, improves itsmanufacturability, and increases its suitability for uses in harshsensing environments. For example, the cane 20 is more difficult to bendwhen compared to fiber. The rigidity makes cane 20 less susceptible tobreakage and losses caused by bending. As is known, optical waveguidescan only be bent to a certain degree (i.e., a bend radius) beforeoptical attenuation becomes intolerable for a given application.Accordingly, standard fibers must be treated with care during packaging,transport, and installation to reduce losses, but this is of lessconcern with cane 20. Cane therefore represents a substantially betterbase media for sensors, such as the inventive sensor configurationsdisclosed herein. As one skilled in the art will appreciate, thethickness of the cladding and/or the cladding-to-core ratio can beoptimized to maximize these benefits for a given application.

[0024] The cane waveguide 20 may alternatively be formed by heating,collapsing, and/or fusing a glass capillary tube to a fiber (not shown)by a laser, filament, flame, etc., as is disclosed in U.S. patentapplication Ser. No. 09/455,865, filed Dec. 6, 1999, entitled“Tube-Encased Fiber Grating”; U.S. Pat. No. 6,422,084, entitled “BraggGrating Pressure Sensor”; U.S. Pat. No. 5,745,626, entitled “Method ForAnd Encapsulation Of An Optical Fiber”; and U.S. Pat. No. 4,915,467,entitled “Method of Making Fiber Coupler Having Integral PrecisionConnection Wells,” all of which are incorporated herein by reference intheir entireties. Other techniques for fusing the tube to the fiberinclude the use of high temperature glass solders, e.g., silica solder(powder or solid), which allow the fiber, the tube, and the solder tobecome integrally fused to a standard fiber, or by the use of laserwelding/fusing techniques.

[0025] Referring to FIG. 2, a side-hole cane waveguide 10 is shownaccording to one embodiment of the present invention. As shown, twosymmetrical holes 12 extend through the cladding 16 substantiallyparallel to the core 14 of a cane waveguide 20. The side-holes 12 arepreferably symmetrically located around the core 14. If the side-holes12 are not symmetrical, the mass distribution in the cane 10 would beunequal. (Unequal mass distribution would effectively cause unbalancedstress distribution within the cane, thus causing the cane to deform orbend away from a linear orientation, which may impede the propagation oflight through the core and increase attenuation). The present inventionmay further include a plurality of symmetrical side-holes, such as 3 or4, symmetrically located around the core. For a 3-4 mm diameter cane 20,the side-holes 12 are preferably 1 mm in diameter, although this is notstrictly necessary and could vary for different applications.

[0026] The side-holes 12 are preferably placed as near to the core aspossible without entering the evanescent field of the core, which is thearea where light propagating through the core region leaks into thecladding 16. The evanescent field may extend several wavelengths fromthe core 14. Locating the side-holes within this field may increaseattenuation, especially if the side-holes 12 are filled with liquid.

[0027] The side-holes 12 of the side-hole cane 10 are preferably formedby boring symmetrical holes into a preform (not shown). An Excimerlaser, a mechanical or other conventional drill, or other knowntechnique may bore the side-holes 12 into the preform. As one skilled inthe art would realize, a “preform” is the structure which issubsequently heated and pulled to form the resulting cane. The side-holecane 10 has a cladding to core ratio of generally 300 to 1, andtherefore the cane preform necessarily should exhibit approximately thesame ratio. The side-holes 12 are drilled into a suitable preform andthen pulled to form the cane with the side-holes.

[0028] An alternative side hole cane manufacturing technique utilizes amethod similar to the PANDA (polarization-maintaining and absorptionreducing) technique used in forming birefringent fibers. As is known,the PANDA technique involves drilling a symmetrical pair of holes oneach side of the core in a VAD (vapor phase axial deposition) preformand then a boron-doped preform (with a different coefficient of thermalexpansion) is inserted into each hole. This composite preform is thendrawn in the usual way to produce a solid fiber in which thestress-producing sectors are formed by the boron-doped MCVD preforms.This methodology can be modified to produce a side-hole cane 10 byinserting hollow silica glass rods in place of the boron-doped preforms.Still other manufacturing techniques are possible. For example, the canewaveguide 20 can be pulled from a solid perform, or formed fromcollapsing a glass tube on a standard fiber, and then drilled to createthe side-holes into the cane 20 by using either an Excimer laser,mechanically or by other known drills. For embodiments incorporating aBragg grating, as disclosed in further detail herein, the grating wouldpreferably be imprinted into the cane after formation of the holes 12.

[0029] The side-holes 12 inherently cause an asymmetry in the stressexerted on the core 14, thus causing an “intrinsic” birefringence,B_(i), which is present even when the side-hole cane 10 is not subjectto an external pressure, P. When external pressure is applied, thispressure is converted into an anisotropic (directionally based) stressin the core region of the fiber, which additionally causes apressure-related birefringence, B_(P). The total birefringence may beexpressed as follows:

B _(total) =B _(i) +B _(P)  Eq. 1

[0030] In a standard cane waveguide 20, different polarizations of lightpropagate at generally the same velocity. However, in a birefringentcane 10, different polarizations of light propagate at differentvelocities due to the asymmetrical stress created on the core. Inreality, what is normally considered a single-mode cane (or fiber) isreally dual-mode due to the fact that there are two possible degeneratepolarization modes of light traveling orthogonally to each other. Morespecifically, a first mode propagates along the x-axis (i.e., the axisdefined by connecting the centers of the side-holes 12) and a secondmode propagates along the y-axis (which is orthogonal to both the x-axisand the axis of the core). A birefringent cane causes light travelingalong the x-axis to have an effective index of refraction of n_(x),which is lower than the effective index of refraction of light travelingalong the y-axis, n_(y). The asymmetric stress of the birefringent canethus effectively causes light traveling along the x-axis to travelfaster than light traveling along the y-axis.

[0031] When a pressure is applied to the birefringent cane, theeffective index of refraction along each axis changes even further. Thusthe total birefringence of the cane, whether intrinsic or by an externalpressure, is related to the indicies of refraction as follows:

B _(total) ∝n _(x) −n _(y)  Eq. 2

[0032] Although the side-hole cane 10 can operate as a pressure sensorin accordance with these principles without the addition of a separatesensing element, it is preferred to incorporate a Bragg grating into theside hole cane structure. Referring to FIG. 5, when a Bragg grating 16is written into the core 14 of the side-hole cane 10, it will reflecttwo wavelengths of light due to the two polarization modes explainedabove. From this reflection, the pressure exerted on the cane may bedetermined as will be explained in more detail below. A Bragg grating18, as is known, is a periodic or aperiodic variation in the effectiverefractive index and/or effective optical absorption coefficient of thecore of a waveguide, and may be formed in accordance with the methodsdisclosed in U.S. Pat. Nos. 4,725,110 and 4,807,950, entitled “Methodfor Impressing Gratings Within Fiber Optics,” to Glenn et al., and U.S.Pat. No. 5,388,173, entitled “Method and Apparatus for Forming AperiodicGratings in Optical Fibers,” to Glenn, which are hereby incorporated byreference. The grating 18 may be in the core 14 and/or in the cladding16 (not shown). Any wavelength-tunable grating or reflective elementembedded, etched, imprinted, or otherwise formed in the cane waveguide10 may be used if desired, all of which constitute “gratings” forpurposes of this disclosure. Further, the reflective element (orgrating) 18 may be interrogated by assessing reflection of lighttherefrom, or by assessing transmission of light therethrough.

[0033] As is known, a Bragg grating reflects a particular wavelength orfrequency of light that is propagating along the fiber core. Theparticular wavelength of light reflected by each Bragg grating, known asthe Bragg reflection wavelength λ_(B), is determined by the Bragggrating spacing, Λ, and the effective index of refraction, n_(eff), asshown in the following equation:

λ_(B)=2n _(eff)Λ  Eq. 3

[0034] As stated previously, in a birefringent fiber, the two orthogonalmodes of light have different index of refractions, n_(x), and n_(y),and travel at different velocities. This imparts a duality to n_(eff),which in turn yields two Bragg reflection wavelengths, λ_(Bx) andλ_(By). This is shown in FIG. 6, which illustrates incident light 11entering into the birefringent region containing a Bragg grating 18. Thetwo orthogonal modes of light at first and second wavelengths arereflected by the Bragg grating 18 back in the direction of the lightsource (not shown) along the x-axis 21 and y-axis 31. The extent of thesplit between the first and second Bragg reflection wavelengths, λ_(Bx)and λ_(By) (Δλ_(B)=|λ_(Bx)−λ_(By)|), is proportional to the pressureincident on the grating, and therefore can be calibrated to allow thebirefringent grating to operate as a pressure sensor. The difference inBragg wavelength Δλ_(B) is given by: $\begin{matrix}\begin{matrix}{{\Delta\lambda}_{B} = {2\left( {n_{x} - n_{y}} \right)\Lambda}} \\{\propto {2\left( {B_{i} + B_{p}} \right)\Lambda}} \\{\propto {{2B_{i}\Lambda} + {2\eta \quad P\quad \Lambda}}}\end{matrix} & {{Eq}.\quad 4}\end{matrix}$

[0035] where η is a coefficient that defines the birefringence of theside-hole cane per unit change in pressure. Because the intrinsicbirefringence B_(i) can be determined without the application ofpressure (i.e., when P=0), and because Λ, η, and Δλ_(B) are known or canbe measured or calculated, the pressure P impingent upon the side-holecane 10 can then be determined.

[0036] One skilled in the art will recognize that temperature alsoaffects the Bragg reflection wavelength, because thermal expansion orcontraction of the grating will affect the grating spacing Λ (see Eq. 3above). However, such thermal effects shift the Bragg reflectionwavelengths λ_(Bx) and λ_(By) by equal amounts, making their differenceΔλ_(B) constant. Accordingly, and in accordance with Eq. 4 above, thedisclosed side-hole cane 10 structure can operate as a pressure sensorwhich is insensitive to temperature and therefore does not requiretemperature compensation.

[0037] However, the disclosed side-hole cane 10 structure can also beused as a temperature sensor which is insensitive to pressure. Forexample, temperature may be determined by measuring at least one peakreflection wavelength as well as its shift as is disclosed in U.S. Pat.No. 5,399,854, entitled “Embedded Optical Sensor Capable of Strain andTemperature Measurement Using a Single Diffraction Grating,” which isincorporated by reference in its entirety. In short, the disclosedside-hole cane 10 structure is elegant in that it allows bothtemperature and/or pressure to be determined through use of a singlesensing element.

[0038] Referring again to FIG. 6, while interrogation of the discloseside-hole cane sensor has been discussed in terms of interpreting theincident light 11 reflected from the sensor (i.e., 21, 31), one skilledin the art will also recognize that the same analysis can be performedby assessing those portion of incident light that are transmitted 13through the sensor.

[0039] Because the two modes of light travel along two orthogonal x- andy-axes, it may be preferable to insert a depolarizer (not shown) nearthe light source (not shown) such that both modes of light interrogatethe sensor to their fullest extent. More specifically, it is preferablein a reflective mode of operation to insert the depolarizer between thelight source and a circulator connected to the photoreceiver such thatthe reflected light from the pressure sensor would not pass back throughthe depolarizer but would proceed directly to the photo receiver. If abroadband light source is used, a passive depolarizer such as a Lyotdepolarizer may be used. If the system uses a narrowband light source,devices such as a polarization scrambler or polarization controller maybe used. As is known, if one polarization axis receives more light thanthe other, that individual mode may dominate, making an assessment ofthe difference between the reflections difficult.

[0040]FIGS. 3 and 4 illustrate various ways in which the disclosedside-hole cane sensor can be attached to various to other pieces of caneto form useful structures. As shown in FIG. 3, the side-hole cane 10 mayadjoin to a cane waveguide by grinding or etching the edges to providetapered, beveled, or angled outer edges 32 or 34. These edges 32 and 34provide a means for mating the cane waveguide 10 with another structuresuch as, for example, another side-hole cane, a standard cane waveguide20, or another structure such as an end cap, a housing, or a largediameter splice (which is described in further detail below). Analternative embodiment for mating the a side-hole cane 10 is disclosedin FIG. 4, in which a central portion of the side-hole cane 10 extendsoutward to form a projection 27 which mates with a receiving pocket 29formed into the structure to be joined. This protrusion 27 or pocket 29may also be joined to other structures, such as a large diameter spliceor standard fiber. Similarly, this projection/pocket arrangement couldbe formed on the outside diameter of the cladding 16 and away from thecore 14, or in the bulk of the cladding 16 between its outside diameterand the core 14. Once mechanically joined, the pieces can be fusedtogether with heat or glued or cemented, or in certain applications notrequiring a firm connection may constitute a press fit. Of course, carewill need to be taken to ensure that the cores 14 of the joinedstructures are aligned and can communicate light with acceptable levelsof loss.

[0041]FIG. 7 depicts an embodiment where a side-hole cane waveguide 10is disposed between two standard cane waveguides 20 used as end caps toform a sensor assembly. The edges may be fusion sealed by a standardfusion arc technique, heated by a resistive heater element toapproximately 1000° C. or more, and/or structurally mated as describedabove if desired. By sealing the side-hole cane 10 to the standard cane20, a first pressure P₁ present within the side-holes 12 can be fixedand hermetically sealed. Thus, in the embodiment illustrated in FIG. 7,the sensor measures a radial pressure P₂ with reference to a sealedvolume of gas or liquid P₁. When radial pressure P₂ compresses the canewaveguide 10, the degree of split between the first and secondwavelengths reflected by the Bragg grating 18 is proportional to theamount of pressure P₂ exerted relative to the internal pressure P₁.While air is preferred for use within the sensor, other inert gases suchas nitrogen or argon, or a liquid, such as silicone or mineral oil,could be used as well in the various embodiments disclosed herein.

[0042]FIG. 8 illustrates another embodiment of a sensor assembly inwhich a side-hole cane 10 is disposed between portions of standardoptical fiber 22. Glass end caps 24 seal gas at pressure P₁ within theside-holes 12, and are preferably fused to the ends of the side-holecane 10. These glass end caps 24 may be formed from slices of standardcane 20 so that the end caps 24 contain a core region for propagatingthe light from the side-hole cane 10. Solid glass or metal end capscould also be used depending on the application at hand, although insuch an application the end caps would need to contain holes foraccommodating the large diameter splices 23 (explained below).Furthermore, the end caps 24 may be ground such that a protrusion (notshown) exists on the opposite side of the end cap 24 from the sideattached to the side-hole cane 10. This protrusion may allow for easierattachment to the large diameter splices 23 or may even be angled toeffectively terminate the propagation of light, depending on theapplication desired. In the embodiment of FIG. 8, the sensor assembly isdual ended and can be multiplexed to other optical devices (e.g., usingwell-known wavelength division multiplexing or time divisionmultiplexing techniques) to form an array. If only a single-ended sensorassembly is desired (e.g., if the sensor assembly constitutes the lastsensor assembly in an array), then alternatively a solid end cap can beused (not shown) thus terminating the light propagation through the core14. To attach a metal end cap to the side-hole cane 10, a metal to glasssealant should be used, many of which exist in the art. As one skilledin the art will recognize, the material for the end cap should besuitable for the intended environment in question. For example, if thesensor apparatus is to be deployed into an oil well to measure thehydrostatic pressures in the production pipe or the well annulus, metalend caps may not be suitable as they may be susceptible to corrosivetemperatures and extreme temperatures that exist downhole.

[0043] Regardless of the assembly to be used in conjunction with theside-hole cane 10, it is generally required to couple the cane and/orits supporting assembly structures to a standard piece to communicationoptical fiber (e.g., 125 micron fiber). To enable this coupling, a largediameter splice 23 may be used, which preferably constitutes a shortsection of a 1 mm-diameter glass capillary tubing which has been heatedto collapse and fuse around the end of a standard fiber 22 to build upits diameter. The increased diameter of the splice 23 provides more bulkmaterial to the end of the standard fiber, which makes it easier tofusion splice that end to the side-hole cane 10, and additionallyconstitute a more rigid connection less susceptible to breaking. Thefiber 22 and splice 23 are preferably joined to the side-hole cane 10 tobring their cores into alignment to ensure minimal optical attenuationat this junction. Fusion splicing of optical waveguides is well known inthe art and the details of such procedures are therefore not furtherdiscussed.

[0044] The sensor apparatus of FIG. 8 can be manufactured in any numberof ways. The end caps 24 can first be connected to the side-hole cane10, and thereafter the splices can be coupled to the resultingstructure. Or, the splices can be attached to the cane 10 first, andthereafter the end caps 24 slid into place by passing the cable 22and/or splice 23 through a hole in the end caps. Either way, the endcaps 24 are sized so as to abut against the ends of the cladding 16 ofthe side-hole cane 10, although the end caps 24 can contain a hole toallow a portion of the cane's core to pass therethrough if necessary.The end caps 24 may be fused, glued or cemented to the side-hole cane 10and to the splice 23 to hermetically seal the holes 12 at a desiredpressure P₁. Alternatively, the end caps can constitute plugs (notshown) which merely fit within and seal off the holes 12. Other steps orconfigurations are possible. As with the embodiment of FIG. 7, gas orliquid can be sealed within the side-holes 12 to create a pressuresensor that, when calibrated, can measure external pressure P₂ relativeto P₁.

[0045] In the embodiments of FIGS. 7 and 8, the internal pressure withinthe side-hole cane 10 is fixed to a desired pressure P₁, which allows anabsolute measurement of external pressure P₂ to be made, in what may bereferred to as an absolute pressure sensor arrangement. However, thebasic design is easily modified so that both the internal pressure P₁and the external pressure P₂ are allowed to vary, such that the sensorassembly measures the differential pressure between the two. Examples ofsuch a differential pressure sensor arrangement are shown in FIGS. 9 and10.

[0046] In FIG. 9, the side-holes 12 of the side-hole cane 10 are sealedat one end by an end cap 24 using the methods described above. The core14 of the side-hole cane 10 is joined to standard fibers 22 using largediameter splices 23, again as discussed previously. To enable theside-hole cane 10 to measure differential pressure, a housing 36surrounds the side-hole cane 10. A first standard fiber 22 enters intothe housing 36 by a hermetic feed-through seal 41, many of which areknown in the art, to join to the core region of the side-hole cane byway of a large diameter splice 23. A protrusion 27 may be formed in theside-hole cane 10 to assist in joining with the splice 23, although thisis not strictly necessary. A second standard fiber 22 joins to the endcap 24 of the side-hole cane 10 by way of a second large diameter splice23 and exits through the housing 36 by a second hermetic feed-throughseal 41. An S-bend curve 42 may further be formed in the standard fiber22 for strain relief, as is well known. The housing 36 defines twopressure chambers 51, 52 for each of the pressures P₁ and P₂respectively, and a ring-shaped partition 38 isolates each chamber 51,52 from each other and otherwise assists in mounting the cane 10 withinthe housing 36. The housing and partitions are preferably made of metal,although other known materials may be used as well, such as glass orceramic materials, depending on the target environment in question. Theuse of glass materials allows the assembly to be fused together, whichmay provide better mechanical stability and corrosion resistance fordeployment in harsh environments such as within an oil/gas well. If madeof metal, the partition 38 joins to the side-hole cane 10 by standardglass-to-metal sealants 43.

[0047] The housing 36 further contains two portholes 40 that enable oneto port the pressures of interest into each respective chamber 51, 52 asdesired. As noted, the difference between the two wavelengths reflectedby the Bragg grating 18 correlates with the difference in pressurebetween P₁ and P₂. Either fluids or gases can be ported into the sensorassembly of FIG. 9. If a liquid pressure is to be measured, but it isnot desired to port liquid directly into the sensor, baffling schemes totransfer pressure from the liquid(s) of interest to the gases residingwithin the chambers in the housing 36 can be accomplished by thetechniques disclosed in U.S. Pat. No. 6,439,055, entitled “PressureSensor Packaging for Harsh Environments,” issued Aug. 27, 2002, which isincorporated herein by reference in its entirety, or by other knowntechniques.

[0048] The present invention also contemplates the use of a housing 36or separate portholes 40 which are configured to effectively isolate oneside-hole 12 from the other, and thereby enabling one to alter thepressure within each hole, effectively a P_(1a) and a P_(1b). As oneskilled in the art will realize, by altering the pressure within eachindividual side-hole 12, the side-hole cane 10 may bend in response tothe asymmetrical stress distribution on the side-hole cane 10 structure,which may or may not be desirable for a given application.

[0049]FIG. 10 illustrates a similar sensor assembly for measuring adifferential pressure, but in this embodiment the housing 36 surroundsonly one side of the side-hole cane 10. The housing includes a singleporthole 40 for porting pressure P₁ into the side-holes 12 of the sensorassembly. The external pressure P₂ is presented directly to the outsideof the side-hole cane 10, as in FIGS. 7 and 8. A ring-shaped partition38 helps to isolate the internal pressure P₁ from the external pressureP₂. This partition 38 joins to the side-hole cane 10 by a glass-to-metalsealant 43 as describe above. In an alternative embodiment, thepartition 38 may also constitute a portion of the housing 36 itself, andmay be directly fused, glued, or cemented to the side-hole cane 10without the need for the additional intervening structures. As in FIG.9, a standard fiber 22 is coupled to the sensor assembly and passesthrough the housing 36 by a hermetic feed-through seal 41. (A fiberstress relief S-bend may also be used but is not shown). This fiber 22then joins directly to the core region of the side-hole cane 10 bymethods described previously. Another standard fiber 22 joins to the endcap 24 by using a large diameter splice 23 as also described previously.As in FIG. 9, the split between the two wavelengths reflected by theBragg grating 18 corresponds to the difference in external pressure P₂and the internal pressure P₁. As realized by one skilled in the art, thehousing 36 may form any shape such that the housing 36 effectivelyisolates the P₁ chambers within the side-holes.

[0050] The absolute pressure sensors in FIGS. 7 and 8 and thedifferential pressure sensors in FIGS. 9 and 10 may be configured andhoused in other ways as well. Several configurations and housing forboth absolute pressure sensors and differential pressure sensors andother techniques which are applicable to the side-hole cane pressuresensors disclosed herein may be found in U.S. Pat. No. 6,422,084,entitled “Bragg Grating Pressure Sensor,” issued Jul. 23, 2002, which isincorporated herein by reference in its entirety. Additionally, to theextent that the sensor assembly are glass, and therefore subject toswelling and chemical attack by moisture ingress, the assemblies (eitherthe side-hole canes and/or any associated housings) can be coated (e.g.,with gold) as is described in U.S. patent application Ser. No.09/494,417, filed Jan. 31, 2000, and entitled “Fluid Diffusion ResistantGlass-Encased Fiber Optic Sensor,” which is incorporated herein byreference in its entirety.

[0051] The disclosed cane-based birefringent sensors have manyadvantages when compared to the fiber-based birefringent sensorsdisclosed earlier. First, because the cane waveguide 10 has a largeouter diameter and cladding when compared to that of a standard opticalfiber 22 (e.g., 125 microns), the cane waveguide 10 does not require aprotective buffer, which simplifies various steps in the manufacturingprocess.

[0052] Second, the large outer diameter D₂ of the cane waveguide 10allows the cane waveguide 10 to be ground, etched, or machined whileretaining the mechanical strength of the cane waveguide 10. Accordingly,cane can be ground to a particular desired diameter, or shaped tofacilitate connection with other components such as in the waysdisclosed earlier. Cane can initially be formed with a 4 mm outsidediameter, and then can be milled to a desired operating diameter(usually after a grating has been written into it), which allowsbirefringent sensors such as those disclosed herein to be tailored to aparticular application. By contrast, fibers, usually 125 microns indiameter, cannot be easily ground, etched or machined without sufferingsignificant mechanical damage. Because it is easy to mechanically work,the cane waveguide 10 may have cross-sectional shapes other thancircular, such as square, rectangular, elliptical, clamshell, octagonal,multi-sided, or any other desired shapes, which may be preferable for agiven application. Such shaping can also allow the core to be madeoff-center with respect to the outside surface of the cladding.

[0053] Third, the mechanical robustness of cane makes it much easier tohouse when compared with fiber-based birefringent sensors. As is known,when working with fiber-based sensors, the fibers themselves often needto be made more mechanically robust to work in a harsh environment, asis disclosed in incorporated reference U.S. Pat. No. 6,422,084. As thatpatent shows, a fiber-based sensor will often need to be housed forprotection, or the fiber themselves will need to be “built up” in sizeby the addition of fused glass capillary tubes to make them suitable foruse in harsh deployment such as down an oil/gas well. However, as FIGS.7, 8, and 10 make clear, housing structures are not required forcane-based sensors (although they can be beneficial as in the embodimentof FIG. 9). Indeed, the cane itself can essentially act as a housing insome applications, and can be directly exposed to the media whosepressure is to be measured. These structural benefits are recognized inthe cane-based birefringent embodiments disclosed herein, and withoutthe need for temperature compensation suggested by the non-birefringentembodiments disclosed in U.S. Pat. No. 6,422,084.

[0054] Fourth, cane-based birefringent sensors allow a greater range ofpressure to be sensed than do fiber-based birefringent sensors, as thethickness of the cane will withstand greater mechanical deformation.Specifically, the cane structure allows for a greater compression rangedue to its relatively small length to diameter aspect ratio whencompared to standard fibers.

[0055] In short the disclosed side-hole cane-based birefringent opticalsensor disclosed herein represents a significant advance overfiber-based birefringent optical sensors or other cane-based opticalsensors.

[0056] The dimensions and geometries for any of the embodimentsdescribed herein are merely for illustrative purposes and, as such, anyother dimensions may be used if desired, depending on the application,size, performance, manufacturing requirements, or other factors, in viewof the teachings herein.

[0057] The grating used in the disclosed embodiments may be tuned bymechanically stressing (i.e. tension, bending) the grating elements, orby varying the temperature of the grating (i.e., using a heater) as isdescribed in U.S. Pat. No. 5,007,705, entitled “Variable Optical FiberBragg Filter Arrangement,” to Morey et al., which is incorporated hereinby reference, or by varying the pressure in each of the cane waveguideholes independently.

[0058] “Cane” as used herein, and as is clear from the foregoingdescription, should not be construed to include structures with claddingdiameters similar to those found in tradition communication opticalfibers (e.g., of 125 micron diameters).

What is claimed is:
 1. A method for measuring an external pressure,comprising: providing an optical cane waveguide having first and secondends comprising: a core comprising a sensor; a cladding surrounding thecore, the cladding comprising an outside surface and containing at leasttwo side-holes parallel to the core; providing a first pressure to theside-holes; providing the external pressure to the outside surface ofthe waveguide; interrogating the sensor with an incident light to createreflected or transmitted light; assessing the reflected or transmittedlight to determine the external pressure relative to the first pressure.2. The method of claim 1, wherein the waveguide has a diameter of atleast 0.3 mm.
 3. The method of claim 1, wherein the waveguide has acladding-to-core ratio of at least 30 to
 1. 4. The method of claim 1,wherein the waveguide is formed of silica glass.
 5. The method of claim1, wherein the sensor comprises a fiber Bragg grating.
 6. The method ofclaim 1, wherein the side-holes are a radial distance of at least 4 λfrom the core.
 7. The method of claim 1, further comprising sealing theside-holes at the first and second ends so that the first pressure isfixed.
 8. The method of claim 7, wherein sealing the side-holescomprises joining the first or second end to an optical cane waveguide.9. The method of claim 7, wherein sealing the side-holes comprisesjoining the first or second end to a cap.
 10. The method of claim 9,wherein the cap includes a core for optically connecting to thewaveguide core.
 11. The method of claim 1, further comprising placing atleast a portion of the waveguide within a housing for porting the firstpressure to the side-holes.
 12. The method of claim 1, furthercomprising placing at least a portion of the waveguide within a housingfor porting the external pressure to the outside surface of thewaveguide.
 13. The method of claim 1, further comprising placing atleast a portion of the waveguide within a housing so that the firstpressure is fixed.
 14. The method of claim 1, further comprising placingat least a portion of the waveguide within a housing so that theexternal pressure is fixed.
 15. The method of claim 1, wherein thesensor reflects a first and second spectral region being respectivelycentered about first and second central wavelengths.
 16. The method ofclaim 15, wherein assessing the reflected or transmitted light includesmeasuring the spectral separation between the first and secondwavelengths.
 17. The method of claim 15, wherein the first and secondwavelengths shift in response to temperature.
 18. The method of claim 1,further comprising assessing the reflected or transmitted light todetermine a temperature of an environment into which the cane waveguideis deployed.
 19. An apparatus for measuring an external pressure,comprising: an optical cane waveguide comprising: a core comprising asensor; a cladding surrounding the core, the cladding comprising anoutside surface and containing at least two side-holes parallel to thecore, wherein the outside surface is exposable to the external pressure;and caps affixed to first and second ends to enclose a first referencepressure within the side-holes.
 20. The apparatus of claim 19, whereinthe waveguide has a diameter of at least 0.3 mm.
 21. The apparatus ofclaim 19, wherein the waveguide has a cladding-to-core ratio of at least30 to
 1. 22. The apparatus of claim 19, wherein the waveguide is formedof silica glass.
 23. The apparatus of claim 19, wherein the sensorcomprises a fiber Bragg grating.
 24. The apparatus of claim 19, whereinthe sensor reflects a first and second spectral region beingrespectively centered about first and second central wavelengths. 25.The apparatus of claim 24, wherein the spectral separation between thefirst and second wavelengths varies in response to the externalpressure.
 26. The apparatus of claim 24, wherein the first and secondwavelengths shift in response to temperature.
 27. The apparatus of claim19, wherein the cap includes a core for optically connecting to thewaveguide core.
 28. An apparatus for measuring a differential pressure,comprising: an optical cane waveguide having a first and second endcomprising: a core comprising a sensor; a cladding surrounding the core,the cladding comprising an outside surface and containing at least twoside-holes parallel to the core, wherein the outside surface isexposable to an external pressure; and a cap affixed to the first end;and a housing affixed to the second end for porting a first pressure tothe side-holes.
 29. The apparatus of claim 28, wherein the waveguide hasa diameter of at least 0.3 mm.
 30. The apparatus of claim 28, whereinthe waveguide has a cladding-to-core ratio of at least 30 to
 1. 31. Theapparatus of claim 28, wherein the waveguide is formed of silica glass.32. The apparatus of claim 28, wherein the sensor comprises a fiberBragg grating.
 33. The apparatus of claim 32, wherein the sensorreflects a first and second spectral region being respectively centeredabout first and second central wavelengths.
 34. The apparatus of claim33, wherein the spectral separation between the first and secondwavelengths varies in response to the differential pressure, thedifferential pressure being the difference between the external pressureand the first pressure.
 35. The apparatus of claim 33, wherein the firstand second wavelengths shift in response to temperature.
 36. Theapparatus of claim 28, further comprising a housing for porting theexternal pressure to the outside surface of the cladding.
 37. Theapparatus of claim 28, further comprising an optical fiber for joiningthe waveguide at the first or second end, wherein the fiber passesthrough the housing at a hermetic seal.
 38. The apparatus of claim 37,further comprising a splice for joining the fiber to the waveguide. 39.The apparatus of claim 37, further comprising a bend in the fiber inbetween the hermetic seal and the waveguide for providing strain relief.40. The apparatus of claim 28, wherein cap includes a core for opticallyconnecting to the waveguide core.